The Manicouagan impact structure is one of the largest impact craters
still preserved on the surface of the Earth. This shuttle view shows
the prominent 70 kilometers (43 miles) diameter, ice-covered annular
lake that fills a ring where impact-brecciated rock has been eroded by
glaciation. The lake surrounds the more erosion-resistant melt sheet
created by impact into metamorphic and
igneous rock types. Shock metamorphic
effects are abundant in the target rocks of the crater floor. Although
the original rim has been removed, the distribution of shock metamorphic
effects and morphological comparisons with other impact structures
indicates an original rim diameter of approximately 100 kilometers (62
miles).